When resistance welding is used to weld two metal parts together, a number of parameters in the welding process may be monitored to consistently provide a quality weld. Typically, the prior art techniques of monitoring weld quality were implemented by attaching or positioning various fragile sensors and their respective leads in direct contact with or in close proximity to the welding electrodes. These monitoring devices and their attendant leads are subject to breakage in industrial environments in which the welding apparatus is required to operate and therefore met with disfavor and were rejected as a means of control because of continual maintainence problems. Examples of the above type of monitoring techniques and associated devices are contained in a publication entitled "Resistance Welding Control and Monitoring" published by the Welding Institute located at Abington Hall, Abington, Cambridge, CB 16A1, United Kingdom, copyrighted 1977.
A control for monitoring a change in resistance across a weld nugget as a weld is formed is disclosed in an application for U.S. patent Ser. No. 197,426 which was filed by the inventor Dennis Jurek on Oct. 16, 1980. The control as disclosed in the Jurek application detects a resistance change across a weld nugget without requiring wires attached to the welding electrodes. The control uses a change in the instants half cycle current pulses in the welding transformer primary winding circuit are extinguished to provide a monitoring signal. One of the disadvantages inherent in the Jurek application control is that the current in the primary winding of the welding transformer, as used to provide a control signal, is not an accurate representation of the welding current in the secondary winding circuit of the welding transformer. While the use of the current in the primary circuit to provide signals possesses advantage that the primary current is considerably less than the welding current in the secondary circuit in any welding apparatus, the current in the primary winding of the welding transformer, is the sum of the welding current in the secondary winding divided by the transformation ratio of the transformer, and the current drawn by the magnetizing branch of the equivalent circuit of the welding transformer. Thus, the current in the primary winding of a welding transformer is not an accurate reflection of the welding current in the welding apparatus.